This invention relates to the removal of mercury from combustion gas streams and more specifically to the use of chemically-treated carbonaceous materials to reduce the emissions of mercury from coal-fired power plants that utilize a hot-side electrostatic precipitator (ESP) to control particulate emissions.
It is well known that mercury is both hazardous and poisonous. Consequently, there is frequently a need to remove it from air streams around industrial processes, such as at chlor-alkali plants, or from the air in dental offices using amalgams, where people may be directly exposed to mercury vapor. Similarly, there is a need to sequester mercury from natural gas and hydrocarbon streams, where it corrodes processing equipment; from wastewater streams, where its discharge can contaminate ecosystems; and from the hot combustion-gas emissions of waste incinerators, where its emission to the environment can methylate and bio-concentrate up the food chain. Each of these gas or liquid streams has different characteristics that make some mercury removal methods effective and appropriate, but makes others ineffective and inappropriate. Consequently, over the years, a multitude of different approaches have been developed for effectively removing mercury species from various streams. These overall approaches include, among others: liquid scrubbing technologies, homogenous gas-phase technologies, metal amalgamation techniques, and processes utilizing various sorbent materials in different application schemes, with sorbents optionally impregnated or reacted with various chemical promoters.
In the past, activated carbons have demonstrated utility for sequestering mercury vapors in some applications. When combined with halogen compounds, the mercury sequestration performance of activated carbons can be improved. In particular, the ability of iodine and iodide impregnations to increase the capacity of granular activated carbons in capturing elemental mercury vapor from air at ambient temperatures has long been known. More recently, bromine-treated activated carbons have shown great efficacy in mercury capture when injected into flue gases as described in U.S. Pat. No. 6,953,494, the disclosure of which is incorporated by reference herein.
A common recent concern is the mercury emitted from coal-fired power plants. It has been estimated, for example, that about 100,000 pounds of mercury are being emitted into the atmosphere annually in the United States from coal-fired power plants. Capturing and isolating this mercury is a very difficult technical problem because the gas volumes to be processed are great, the concentrations of the mercury in the gas are relatively low, and the gas temperatures are high. Also, many other complicating compounds are present in the flue gas and multiple mercury species have to be sequestered.
Hot-Side ESPs have been used in many applications where the resistivity of the fly ash or dust make it difficult to collect in a cold-side ESP. About 10% of the U.S. utility boilers are of the hot-side design. Hot-side ESPs operate at temperatures typically between 230° C. and 455° C. (450° F. and 850° F.), as compared to the typical cold-side ESP operating temperature of 120° C. and 205° C. (250° F. to 400° F.). The hot-side ESP gets its name from the fact that the control device is positioned before the air preheater, which is the hot side of the air preheater. The operation of the ESP at elevated temperatures tends to reduce the ash resistivity and make it easier to capture.
The unburned carbon in fly ash loses most of its mercury removal capacity above 230° C. (450° F.). Thus, there is very little native mercury removal by the unburned carbon in hot-side ESPs. Similarly, plain powdered activated carbon (PAC) has little to no mercury removal capacity above these temperatures and, therefore, has little to no value in mercury control in these applications.
Thus, there is a need for new means for effectively and economically controlling utility mercury emissions, particularly for use in a hot side ESP.